Water-absorbing polymers having interstitial compounds, a process for their production, and their use

ABSTRACT

The invention relates to absorbents for water and aqueous liquids, which absorbents are based on water-swellable, yet water-insoluble polymers wherein zeolites high in silicon have been incorporated ionically and/or as a result of mechanical inclusion.

[0001] The invention relates to absorbents for water and aqueousliquids, which absorbents are based on water-swellable, yetwater-insoluble polymers wherein zeolites high in silicon have beenincorporated ionically and/or as a result of mechanical inclusion.

[0002] Commercially available superabsorbing polymers essentially arecrosslinked polyacrylic acids, crosslinked starch/acrylic acid graftcopolymers, crosslinked hydrolyzed starch/acrylonitrile graftcopolymers, crosslinked poly(maleic anhydride-co-isobutylene), ormixtures of various of the above-mentioned crosslinked polymers, whereinthe carboxylic groups have been subjected to partial neutralization withsodium and/or potassium ions. Such polymers find use e.g. in hygienearticles capable of absorbing body fluids such as urine or menstrualfluid or in absorbent pads in packagings for foodstuffs where theyabsorb large amounts of aqueous liquids and body fluids such as urine orblood with swelling and formation of hydrogels. Furthermore, theabsorbed amount of liquid must be retained under a pressure typical ofuse. During the further technical development of superabsorbingpolymers, the pattern of requirements to be met by these products haschanged significantly over the years.

[0003] To date, the development of superabsorbers has been forcedparticularly with respect to the amount of absorbed liquid and pressurestability. Such crosslinked polymer products based on monomerscontaining acid groups are obtained by using one or more primarycrosslinkers and/or one or more secondary crosslinkers and exhibit acombination of properties, namely, high retention, high absorption underpressure, low solubles, rapid absorption of liquid, and highpermeability in the swollen state, which has not been achieved so far.When used in hygiene articles, these crosslinked polymer products havethe advantage that secreted fluids, once absorbed by the polymerproduct, can no longer contact the skin. Thus, skin lesions such asdiaper dermatitis can largely be avoided. Such comfort can even beincreased by absorbing malodorous compounds.

[0004] According to Römpp Chemie Lexikon, the content of urinecomponents and thus, of malodorous compounds, is subject tophysiological fluctuations; also, particular substances are secreted atconcentrations varying within a daily period, so that more precise dataon the urine composition invariably are related to the so-called 24 hoururine which, in a healthy adult, contains e.g. urea (average 20 g), uricacid (0.5 g), creatinine (1.2 g), ammonia (0.5 g), amino acids (2 g),proteins (60 mg), reducing substances (0.5 g, about 70 mg of which areD-glucose or urine sugar), citric acid (0.5 g) and other organic acids,as well as certain vitamins (C, B₁₂ etc.). The following inorganic ionsare present: Na⁺ (5.9 g) , K⁺ (2.7 g) , NH₄ ⁺ (0.8 g) , Ca²⁺ (0.5 g),Mg²⁺ (0.4 g); Cl⁻ (8.9 g), PO₄ ³⁻ (4.1 g), SO₄ ⁻² (2.4 g). The drycontent is between 50 and 72 g. Inter alia, alkylfurans, ketones,lactones, pyrrole, allyl isothiocyanate, and dimethyl sulfone have beenrecognized as volatile components of urine. Most of the volatilecomponents are molecules having a molar mass below about 1000 g/mol anda high vapor pressure.

[0005] Volatile components of urine have also been investigated by,inter alia, A. Zlatkis et al. (Anal. Chem. Vol. 45, 763ff.). It is alsowell-known that consumption of asparagus results in an increase of theconcentration of organic sulfur-containing compounds in human urine (R.H. Waring, Xenobiotika, Vol. 17, 1363ff.). In patients who are subjectto specific diets and/or ingest specific medications, or in elderlyindividuals with decreasing kidney function, the urine may includemalodorous substances. Patients suffering from urine incontinence havean increased secretion of ureases which convert the urea contained inurine, thereby liberating toxic ammonia. Also, a pathological change iswell-known which is referred to as fish smell syndrome. It results froman increased secretion of quaternary ammonium compounds. Also, menstrualfluid may acquire an unpleasant odor. Among other things, this odor isproduced by microbial degradation of secreted proteins. Typical odoroussubstances in menstrual fluid and the smells produced by degradation ofblood components are not substantially different from the smell ofcomponents occurring in urine. In this case as well, low molecularweight compounds having a molar weight of less than 1000 g/mol areinvolved. Predominantly, nitrogen-containing heterocycles such aspyrrole, pyridine and derivatives thereof may be mentioned. Furthermore,those smells liberated by foodstuffs may be mentioned, e.g. the smell offish (amines).

[0006] The odorous components in vaginal secretions and menstruationfluid have been investigated by G. Huggins and G. Preti (ClinicalObstetrics and Gynecology, Vol. 24, No. 2, June 1981, 355-377), wherelow molecular weight substances having a molar weight below 500 g/molhave been found. Fatty acids (e.g. butyric acid, isovaleric acid) andsome aromatic compounds such as pyridine, indole and thymine may beemphasized, which particularly contribute to unpleasant odors. Theamount of volatile fatty acids varies over the time period of themenstrual cycle (Human Vaginal Secretions: Volatile Fatty Acid Content,Richard P. Michael, R. W. Bonsall, Patricia Warner, Science, Dec. 27,1974, 1217-1219). Amines have not been found in vaginal secretions andmenstruation fluid. This is because the pH value of the secretion in ahealthy female patient is in the acidic range where, at most, ammoniumsalts are present which are non-volatile. It is only in pathologicalconditions where proteins increasingly can be converted to amines bybacterial degradation, which may enter the vapor space in case of asimultaneous increase of the pH value.

[0007] Previous approaches of achieving an odor reduction inincontinence products and Ladies' hygiene products are based on reducingthe concentration of free ammonia. Basically, there are two approachesto this end: preventing additional production of ammonia from ureadegradation by suitable urease inhibitors (A. Norberg et al.,Gerontology, 1984, 30, 261ff.), or by protonating free ammonia andbinding thereof in the form of a carboxylate ammonium salt. This methodis disadvantageous in that essentially, merely ammonia and othernitrogen-containing components can be controlled. Malodorous compoundslacking basic groups, e.g. thiols, are still capable of entering thevapor space.

[0008] It is well-known to those skilled in the art that zeolites havehigh adsorptive roperties.

[0009] Zeolites mostly are synthetic compounds comprised of siliconoxide, aluminum oxide and a number of metal ions. Their composition isM₂O_(z).Al₂O₃.xSiO₂.yH₂O wherein M=uni- or multivalent metal, H,ammonium, etc., z=valency, x=from 1.8 to about 12, and y=from 0 to about8. Structurally, zeolites are comprised of SiO₄ and AlO₄ tetrahedronslinked via oxygen bridges, thereby forming a channel system of equallystructured and equally large interconnected cavities. Zeolites are namedaccording to their pore openings, e.g. zeolite A (4.2 Å), zeolite X (7.4Å). When heated, most zeolites release their water continuously, withoutaltering their crystal structure. In this way, they are capable ofaccommodating other compounds, acting as e.g. catalysts or ionexchangers. Furthermore, zeolites exhibit a screening effect byincorporating molecules having a smaller cross-section than the poreopenings in the lattice channel system. Larger molecules are excluded.Cations are required to balance the negative charge of the AlO₄tetrahedrons in the alumosilicate skeleton.

[0010] Inter alia, the synthesis of zeolites has been describedextensively in: Zeolite Synthesis, ACS Symposium Series 398, Eds. M. L.Ocelli and H. E. Robson (1989) pp. 2-7. The synthesis of hydrophobiczeolites having a silicon dioxide/aluminum oxide ratio in the skeletonof >100, high hydrothermal stability and resistance to aqueous alkalinesolutions is disclosed in the patent application DE 195 32 500 A1. Thezeolites have a grain size of markedly less than 150 μm.

[0011] The patent document U.S. Pat. No. 4,795,482 teaches the use ofhydrophobic zeolites to suppress and avoid organic odors. The reductionof odors was measured using headspace gas chromatography.

[0012] It is well-known from the patent applications WO 91/12029 and WO91/12031 that the hydrophobic zeolites described in U.S. Pat. No.4,795,482 or produced in a similar way can be used in combination withsuperabsorbers, where the zeolite is “essentially” bound to thesuperabsorber. The composites thus obtained are used in hygiene articlessuch as diapers or liners. The mixture is produced by mixing thesuperabsorber with the zeolite in dry condition. Water is subsequentlyadded, where aggregation of the particles has been observed (WO91/12031). Following a drying step, the mixture can be incorporated inhygiene products. In the patent application WO 91/12029, the zeolite isdispersed in water together with a binder and coated onto thesuperabsorber in a coating process where at least 20% zeolite, relativeto the superabsorber, is to be used.

[0013] Both of these procedures are disadvantageous in that binding ofthe zeolite material to the polymer is exceedingly weak, and separationand demixing of superabsorber and zeolite may occur even at lowmechanical stress on the composite. Such mechanical stress occurs e.g.when conveying a superabsorber and/or an absorbent article includingsuperabsorbing polymers. In addition to demixing, problems of handlingexceedingly fine particles also arise. Also, when subjecting thesuperabsorber to a secondary treatment with aqueous dispersions possiblycontaining binders, damage to the superabsorber structure and theassociated swelling properties must be expected. The high percentage ofnon-swellable zeolite material in the superabsorber composite representsan additional limitation to the pattern of properties.

[0014] It is well-known from the patent applications EP 0,811,387 A1 andEP 0,811,390 A1 that zeolites having a silicon dioxide/aluminum oxideratio of from 1 to 5 can be used as odor absorbents in liners. Theproducts produced according to the above document were subjected to apractical test, and the used products were rated in an olfactory testpanel with test persons. The dry mixtures described in theabove-mentioned patent applications readily undergo demixing. Inaddition, the amounts of required zeolite as taught in the above patentapplications are exceedingly high, having a disadvantageous effect onthe wearing comfort of hygiene articles.

[0015] The present invention therefore is based on the object ofproviding a polymer capable of absorbing water and aqueous liquids,which polymer has a substance by means of which malodorous organiccompounds such as occurring e.g. in urine or other fluids secreted fromthe body are bound, and wherein

[0016] malodorous materials released into the vapor space during use aremarkedly reduced;

[0017] a virtually uniform distribution of deodorant substance in theabsorbent is present;

[0018] demixing in the condition prior to and during use is avoided asmuch as possible;

[0019] the absorbent has good retention properties and swellingproperties under pressure; and

[0020] the deodorant modification is ensured using amounts of deodorantsubstance as low as possible.

[0021] The present invention is also based on the object of providing aprocess for producing said deodorant absorbent wherein

[0022] in particular, problems with mixing of dry substances differingsubstantially in their particle size, such as granulates and powders,are avoided;

[0023] no dust is formed; and

[0024] aggregation of the particles during production is avoided.

[0025] According to the invention, said object is accomplished by meansof an absorbent polymer constituted of crosslinked, monoethylenicallyunsaturated, partially neutralized monomers bearing acid groups, whichpolymer has zeolites high in silicon bound or incorporated in an ionicalor mechanical fashion.

[0026] In the meaning of the invention, “high in silicon” means that thesilicon dioxide/aluminum oxide ratio is >10, preferably >20, morepreferably >50, and even more preferably >100. A silicondioxide/aluminum oxide ratio of >500 is particularly preferred.

[0027] In the meaning of the invention, “crosslinked” means that thepolymer is crosslinked and/or surface-crosslinked.

[0028] The zeolites to be used according to the invention aredealuminized, hydrophobic (organophilic) zeolite variants having asilicon dioxide/aluminum oxide ratio in their skeletons of >10,preferably >20, more preferably >50, with >100 being particularlypreferred. A ratio of >500 is most preferred. The amount to be used is0.01-10 wt.-%, preferably 0.1-5 wt.-%, and more preferably 0.70-3 wt.-%,relative to the total amount of absorbent. For example, such zeolitesare traded by Degussa AG under the trade name Flavith® or by UOP underthe designation of Abscents®. Flavith® is characterized in more detailin the KC-CZ 42-1-05-1098 T&D product data sheet. Said product datasheet is hereby incorporated by reference and thus represents part ofthe disclosure.

[0029] Various processes are possible for polymerizing the polymer ofthe invention optionally having superabsorbent properties, e.g. bulkpolymerization, solution polymerization, spray polymerization, inverseemulsion polymerization, and inverse suspension polymerization.Preferably, a solution polymerization is performed using water assolvent. The solution polymerization may be conducted in a continuous orbatchwise fashion. The patent literature includes a broad spectrum ofpossible variations with respect to concentration conditions,temperatures, type and amount of initiators and of secondary catalysts.Typical processes have been described in the following patentspecifications: U.S. Pat. No. 4,286,082; DE 27 06 135, U.S. Pat. No.4,076,663, DE 35 03 458, DE 40 20 780, DE 42 44 548, DE 43 23 001, DE 4333 056, DE 44 18 818. These disclosures are hereby incorporated byreference and thus represent part of the disclosure.

[0030] The unsaturated acid group-containing monomers to be usedaccording to the invention are, e.g. acrylic acid, methacrylic acid,crotonic acid, isocrotonic acid, maleic acid, fumaric acid, itaconicacid, vinylacetic acid, vinylsulfonic acid, methallylsulfonic acid,2-acrylamido-2-methyl-1-propanesulfonic acid, as well as the alkaliand/or ammonium salts thereof. It is preferred to use acrylic acid andits alkali and/or ammonium salts and mixtures thereof. Furthermore, itis also possible to use monomers being hydrolyzed to form acid groups aslate as subsequent to the polymerization as is possible e.g. withnitrile groups.

[0031] In order to modify the polymer properties, up to 30 wt.-% ofother comonomers soluble in the aqueous polymerization batch, such asacrylamide, methacrylamide, acrylonitrile, (meth)allyl alcoholethoxylates, and mono(meth)acrylic acid esters of alcohols orethoxylates can optionally be used.

[0032] Minor amounts of crosslinking monomers having more than onereactive group in their molecules are copolymerized together with theabove-mentioned monomers, thereby forming partially crosslinked polymerproducts which are no longer soluble in water but merely swellable. Bi-or multifunctional monomers, e.g. amides such as methylenebisacryl- or-methacrylamide, or ethylenebisacrylamide may be mentioned ascrosslinking monomers, and also, allyl compounds such as allyl(meth)acrylate, alkoxylated allyl (meth)acrylate reacted preferably withfrom 1 to 30 mol of ethylene oxide, triallyl cyanurate, maleic aciddiallyl ester, polyallyl esters, tetraallyloxyethane, triallylamine,tetraallylethylenediamine, allyl esters of phosphoric acid orphosphorous acid, and also, crosslinkable monomers such as N-methylolcompounds of unsaturated amides like methacrylamide or acrylamide andthe ethers derived therefrom, as well as esters of polyols andalkoxylated polyols, such as diacrylates or triacrylates, e.g.butanediol or ethylene glycol diacrylate, polyglycol di(meth)acrylates,trimethylolpropane triacrylate, di- and triacrylate esters oftrimethylolpropane preferably oxyalkylated (ethoxylated) with 1 to 30mol alkylene oxide, acrylate and methacrylate esters of glycerol andpentaerythritol, and of glycerol and pentaerythritol preferablyoxyethylated with 1 to 30 mol ethylene oxide. It is preferred to usetriallylamine, acrylates of polyhydric alcohols or alkoxylates thereof,and methallyl alcohol acrylates or alkoxylates thereof. The ratio ofcrosslinking monomers is from 0.01 to 3.0 wt.-%, preferably from 0.05 to2.0 wt.-%, and more preferably from 0.05 to 1.5 wt.-%, relative to thetotal monomers.

[0033] The acidic monomers preferably are subjected to neutralization.The neutralization can be performed in various ways. On the one hand,according to the teaching of U.S. Pat. No. 4,654,039, the polymerizationmay be conducted directly with the acidic monomers, with neutralizationbeing effected subsequently in the polymer gel. This patentspecification is hereby incorporated by reference and thus representspart of the disclosure. On the other hand and preferably, the acidicmonomer components are neutralized to 20-95%, preferably 50-80% prior topolymerization, in which case they are present as sodium and/orpotassium and/or ammonium salts at the time polymerization is begun. Itis preferred to use those bases for neutralization which do notadversely affect the subsequent polymerization. It is preferred to usesodium or potassium hydroxide solution and/or ammonia, with sodiumhydroxide solution being particularly preferred; addition of sodiumcarbonate, potassium carbonate or sodium bicarbonate may have anadditional positive effect as taught in U.S. Pat. Nos. 5,314,420 and5,154,713. Before initiating the polymerization in this adiabaticsolution polymerization, the partially neutralized monomer solution iscooled to a temperature of below 30° C., preferably below 20° C. Thesepatent specifications are hereby incorporated by reference and thusrepresent part of the disclosure. In the other processes mentioned,other temperatures are also well-known and conventional according to thestate of the art.

[0034] The polymer products of the invention may contain water-solublepolymers as a basis for grafting in amounts up to 40 wt.-%. Inter alia,these include partially or completely saponified polyvinyl alcohols,starch or starch derivatives, cellulose or cellulose derivatives,polyacrylic acids, polyglycols, or mixtures thereof. The molecularweights of the polymers added as basis for grafting must be adapted tothe circumstances of the polymerization conditions. In the event of anaqueous solution polymerization, for example, it may be necessary forviscosity reasons to employ low or medium molecular weight polymersonly, whereas this factor plays a minor role in a suspensionpolymerization.

[0035] In addition to polymers obtained by crosslinking polymerizationof partially neutralized acrylic acid, those are preferably used whichadditionally contain components of graft-polymerized starch, or ofpolyvinyl alcohol.

[0036] The polymerization process of the invention can be initiated byvarious conditions, e.g. by irradiating with radioactive,electromagnetic or ultraviolet radiation, or by a redox reaction of twocompounds, e.g. sodium hydrogen sulfite with potassium persulfate, orascorbic acid with hydrogen peroxide. The thermally induceddecomposition of a so-called free-radical initiator such asazobisisobutyronitrile, sodium peroxodisulfate, t-butyl hydroperoxide,or dibenzoyl peroxide may also be used as initiation of polymerization.Furthermore, a combination of some of the above-mentioned methods ispossible.

[0037] In principle, the polymer products are produced according to twomethods: According to the first method, the partially neutralizedacrylic acid is converted to a gel by means of free-radicalpolymerization in aqueous solution and in the presence of crosslinkersand optional polymer additives, which gel is subsequently crushed anddried until a powdered, flowable state is reached, milled, and screenedto the desired particle size. The solution polymerization may beconducted in a continuous or batchwise fashion. The patent literatureincludes a broad spectrum of possible variations with respect toconcentration conditions, temperatures, type and amount of initiators,as well as a variety of secondary crosslinking options. Typicalprocesses have been described in the following patent specifications:U.S. Pat. No. 4,076,663; U.S. Pat. No. 4,286,082; DE 27 06 135, DE 35 03458, DE 35 44 770, DE 40 20 780, DE 42 44 548, DE 43 23 001, DE 43 33056, DE 44 18 818. These documents are hereby incorporated by referenceand thus represent part of the disclosure.

[0038] The inverse suspension and emulsion polymerization process mayalso be used to produce the polymer products. In these processes, anaqueous, partially neutralized solution of acrylic acid is dispersed ina hydrophobic organic solvent using protective colloids and/oremulsifiers, and the polymerization is initiated using free-radicalinitiators. The crosslinkers are either dissolved in the monomersolution and metered together with same or added separately andoptionally subsequently. The optionally present polymeric grafting basesare added via the monomer solution or by directly placing in the oilphase. Subsequently, the water is removed azeotropically from themixture, and the polymer product is filtrated and optionally crushed anddried until a powdered, flowable state is reached, milled, and screenedto the desired particle size.

[0039] Using the process of subsequent surface crosslinking, the polymerproducts according to the invention can be improved in their pattern ofproperties, particularly in their absorption of liquid under pressure,so that the well-known phenomenon of “gel blocking” is suppressed, whereslightly swelled polymer particles adhere to each other, therebyimpeding further absorption of liquid and distribution of liquid e.g.within the diaper. In this secondary crosslinking, the carboxyl groupsof the polymer molecules are crosslinked at the surface of thesuperabsorber particles at elevated temperature using crosslinkingagents. Methods of secondary crosslinking have been described in severalwritten specifications, e.g. in DE 40 20 780, EP 317,106 and WO 94/9043.

[0040] According to the invention, all those secondary crosslinkingagents known to a person skilled in the art from U.S. Pat. No.5,314,420, page 8, lines 3-45, may be used advantageously in combinationwith a primary crosslinker or a combination of crosslinkers. Theabove-mentioned documents are hereby incorporated by reference and thusrepresent part of the disclosure. As a rule, these compounds contain atleast two functional groups capable of reacting with carboxylic acid orcarboxyl groups. Alcohol, amine, aldehyde, and carbonate groups arepreferred and also, crosslinker molecules having multiple differentfunctions are employed. Preferably, polyols, polyamines,polyaminoalcohols, polyepoxides, and alkylene carbonates are used. Inparticular, one of the following secondary crosslinking agents is used:ethylene glycol, diethylene glycol, triethylene glycol, polyethyleneglycol, glycerol, polyglycerol, propylene glycol, diethanolamine,triethanolamine, polypropylene glycol, block copolymers of ethyleneoxide and propylene oxide, sorbitan fatty acid esters, ethoxylatedsorbitan fatty acid esters, trimethylolpropane, ethoxylatedtrimethylolpropane, pentaerythritol, ethoxylated pentaerythritol,polyvinyl alcohol, sorbitol, ethylene carbonate, propylene carbonate. Itis particularly preferred to use polyols and ethylene carbonate assecondary crosslinking agents. The secondary crosslinking agent isemployed in an amount of from 0.01 to 30 wt.-%, preferably 0.1-10 wt.-%,relative to the polymer to be subjected to secondary crosslinking.

[0041] Prior to secondary crosslinking, the polymer preferably is dried,milled, screened for the respective grain fraction favorable inapplication-technical terms, and subsequently fed into the secondarycrosslinking reaction. In some cases, however, it has proven beneficialto add the secondary crosslinkers at an early stage prior to drying thepolymer gel or prior to crushing the partially or predominantly driedpolymer. Secondary crosslinking to be performed according to theinvention has been described in U.S. Pat. No. 4,666,983 and DE 40 20780. These documents are hereby incorporated by reference and thusrepresent part of the disclosure. Advantageously, the secondarycrosslinker frequently is added in the form of a solution in water,organic solvents or mixtures thereof, particularly in those cases wherelow amounts of secondary crosslinking agent are used. Suitable mixingapparatus for applying the secondary crosslinking agent are, e.g.,Patterson-Kelley mixers, DRAIS turbulence mixers, Lödige mixers, Rubergmixers, screw mixers, pan mixers, and fluid-bed mixers, as well ascontinuously operated vertical mixers wherein the powder is mixed at arapid frequency using rotating knives (Schugi mixer). Once the secondarycrosslinker has been mixed with the pre-crosslinked polymer, heating totemperatures of from 60 to 250° C., preferably from 135 to 200° C., andmore preferably from 150 to 185° C. is effected in order to perform thesecondary crosslinking reaction. The time period for additional heatingis limited by that point where the desired pattern of properties of thepolymer product is destroyed as a result of heat damage.

[0042] Depending on the type of use, various screening fractions areemployed for processing the polymer products, e.g. between 100 and 1000μm and preferably between 150 and 850 μm for diapers. In general, thisgrain fraction is produced by milling and screening prior to and/orsubsequent to secondary crosslinking.

[0043] In the polymer product of the invention for absorbing water oraqueous liquids, the zeolite component can be extracted by the liquid tobe absorbed to only a lesser extent, or, in the dry state, undergodemixing to only a lesser extent. Surprisingly, it has been found thatthe zeolites not even partly lose their ability of absorbing odors as aresult of the intimate linkage with the crosslinked polymer bearing acidgroups. In this way, the vapor space concentration of malodoroussubstances is effectively reduced. The deodorant substances are appliedfrom a suspension, for example. In this way, any dust problems duringmanufacturing are avoided. It has also been found that the zeolitesneither lose their ability of absorbing odors when applied from anaqueous suspension. Furthermore, it has been found that the otherquality criteria relevant for polymers having superabsorbent properties,namely, high retention and absorption against pressure, are notadversely affected by applying zeolite.

[0044] The polymer product of the invention is excellently suited forincorporating active substances, and when used, these active substancescan optionally be released in a controlled fashion. By incorporation inthe polymer products of the invention, the stability of sensitive activesubstances is markedly improved.

[0045] The present invention is also directed to a process for producingthe absorbent polymer products of the invention.

[0046] According to the process of the invention, the absorbent polymerproduct of the invention is produced by:

[0047] free-radical polymerization of an aqueous solution ofethylenically unsaturated, optionally partially neutralized monomersbearing acid groups and crosslinking monomers according to the processof solution or suspension polymerization to form a hydrogel;

[0048] optional isolation;

[0049] crushing, followed by drying, milling;

[0050] optional screening; and

[0051] surface crosslinking;

[0052] wherein the zeolite high in silicon is added to the polymerproduct during its surface crosslinking at the latest.

[0053] Preferably, the zeolite is added in suspension.

[0054] In the meaning of the invention, “high in silicon” means that thesilicon dioxide/aluminum oxide ratio is >10, preferably >20, morepreferably >50, and even more preferably >100. A silicondioxide/aluminum oxide ratio of >500 is particularly preferred.

[0055] The zeolite is employed in suspension. A preferred liquid phaseis water, but mixtures of water and organic solvents are also used.

[0056] According to the invention, the addition of zeolite can beeffected at various process stages in the production of the powderedpolymer product, as illustrated below. By applying the zeolite from anaqueous suspension onto the polymer product prior to or during one ofthe process steps in the production thereof, particularly effectivebinding between the odor-absorbing component and the polymer product isachieved.

[0057] In a preferred embodiment of the process according to theinvention, the zeolite is added directly to the aqueous monomer solutionprior to polymerization. In case the absorbent is produced by suspensionpolymerization, it is also possible to precharge all or part of thezeolite in the oil phase and meter the monomer solution thereto. Whereonly part of the zeolite is precharged, the remainder is to be added viathe monomer solution.

[0058] In another preferred embodiment of the process according to theinvention, the zeolite is applied onto the crushed polymer gel in theform of a suspension in water or an organic solvent or mixtures thereof.

[0059] Furthermore, the polymer gel preferably is subjected to at leastpartial drying and the zeolite subsequently is applied onto the powderin the form of a suspension in water or an organic solvent or mixturesthereof. The resulting product can be dried directly as such andsubjected to surface crosslinking.

[0060] In another preferred embodiment of the process according to theinvention, the zeolite is employed in the processing step of secondarycrosslinking. Suitable mixing apparatus for applying the secondarycrosslinking agent are e.g. Patterson-Kelley mixers, DRAIS turbulencemixers, Lödige mixers, Ruberg mixers, screw mixers, pan mixers, andfluid-bed mixers, as well as continuously operated vertical mixerswherein the powder is mixed at a rapid frequency using rotating knives(Schugi mixer).

[0061] It is also preferred to incorporate the zeolite at various stagesof the production process of the absorbent polymers so as to optimizethe effect of the zeolite and utilize synergies.

[0062] According to another process of the invention, the absorbentpolymer product of the invention is produced by:

[0063] free-radical polymerization of an aqueous solution ofethylenically unsaturated, optionally partially neutralized monomersbearing acid groups and crosslinking monomers according to the processof solution or suspension polymerization to form a hydrogel;

[0064] optional isolation;

[0065] crushing, followed by drying, milling;

[0066] optional screening;

[0067] wherein the zeolite high in silicon is added to the polymerproduct at a stage where the water content thereof is at least 10 wt.-%.

[0068] Preferably, the addition of zeolite high in silicon is effectedin suspension.

[0069] According to the invention, the water content must not be reducedbelow 10 wt.-% before the zeolite high in silicon is added.

[0070] Preferably, the water content must not be reduced below 30 wt-%,more preferably not below 50 wt.-%, and even more preferably not below65 wt.-% before the zeolite high in silicon is added.

[0071] The addition of the zeolite to the polymer product preferably iseffected using a suspension.

[0072] Using the process according to the invention, absorbent polymersare obtained wherein the zeolite is incorporated in the syntheticpolymer in such a way that the cannot be removed completely from thepolymer product even after mechanical stressing e.g. in a ball mill at95 rpm for 6 minutes. In the process of the invention, less than 80% andgenerally 40-60% of the total amount of zeolite applied onto the polymerproduct is removed after such stressing.

[0073] Compared to powdered absorbents including no zeolite, the polymerproducts of the invention exhibit improved absorption of malodorouscompounds.

[0074] The polymer products find use e.g. in hygiene articles capable ofabsorbing body fluids such as urine, or in the packaging sector, e.g.meat and fish products, where they absorb large amounts of aqueousliquids and body fluids such as urine, blood, or meat juice, withswelling and formation of hydrogels. Therefore, the present invention isalso directed to these uses.

[0075] The polymer products of the invention are incorporated directlyas powders in constructions for absorbing liquids, or previously fixedin foamed or non-foamed sheet materials. For example, such constructionsfor absorbing liquids are diapers for babies, incontinence articles orabsorbent inserts in packaging units for foodstuffs. In the absorbentpolymer product according to the invention, binding of the zeolite tothe polymer obviously is so strong that even under mechanical stress,e.g. when conveying the absorbent polymer product, substantialseparation and demixing of the polymer and zeolite cannot be observedand thus, in particular, problems of handling exceedingly fine particlesdo not occur.

[0076] Further processing of the polymer product according to theinvention is advantageous because, according to prior art, separate anduniform dosage of superabsorber and zeolite, particularly with smallamounts of zeolite, cannot be achieved. The polymer product according tothe invention, which allows easy dosing, ensures a constantconcentration of polymer product with superabsorbent properties anddeodorant component in absorbent articles such as liners.

[0077] Moreover, the absorbents of the invention were found to beexcellently suited for incorporating active substances. The stability ofsensitive active substances, e.g. with respect to oxidative degradation,is substantially improved as a result of incorporation in the absorbentsof the invention.

[0078] Furthermore, the polymer products according to the invention finduse in plant breeding and in pest control in agriculture. In plantbreeding, the polymer products in the vicinity of plant roots providefor sufficient supply of liquid and previously incorporated nutrientsand are capable of storing and releasing same over a prolonged period oftime.

[0079] In pest control, the polymer product can incorporate singleactive substances or a combination of multiple active substances whichin use are released in a controlled fashion in terms of time and amount.

[0080] The invention will be illustrated in the following Examples.These illustrations merely are given by way of example and do not limitthe general idea of the invention.

[0081] Production and properties of the polymer products according tothe invention will be explained. Furthermore, the test methods andprocedures used to determine the characteristics of the polymers withsuperabsorbent properties will be described.

Test Methods Test Method 1

[0082] The retention is determined according to the tea bag method andis given as mean value of three measurements. About 200 mg of polymerproduct is welded in a tea bag and immersed in a 0.9% NaCl solution for30 minutes. The tea bag is subsequently centrifuged in a centrifuge (23cm in diameter, 1400 rpm) for 3 minutes and weighed. A tea bag having nowater-absorbing polymer is run as a blank.${Retention} = {\frac{{{Final}\quad {weight}} - {Blank}}{{Initial}\quad {weight}}\left\lbrack {g\text{/}g} \right\rbrack}$

Test Method 2:

[0083] Liquid Absorption Under Pressure (AAP Test, EP 0,339,461)

[0084] The absorption under pressure (pressure load 50 g/cm²) isdetermined according to the method described in EP 0,339,461, page 7.This document is hereby incorporated by reference and thus representspart of the disclosure. About 0.9 g of superabsorber is weighed in acylinder having a screen bottom. The uniformly spread superabsorberlayer is loaded with a piston exerting a pressure of 50 g/cm². Thepreviously weighed cylinder then is placed on a glass filter platesituated in a tray containing a 0.9% NaCl solution, the liquid level ofwhich precisely corresponds to the height of the filter plate. Afterallowing the cylinder unit to absorb 0.9% NaCl solution for 1 hour, itis reweighed, and the AAP is calculated as follows:

[0085] AAP=Final weight (cylinder unit+superabsorber)−Initial weight(cylinder unit+soaked superabsorber)/Initial weight of superabsorber

Test method 3

[0086] Determination of the Absorption of Malodorous Compounds

[0087] 0.1 g of powdered absorbent is added with 2 ml of an aqueoussolution (including 5 wt.-% ethanol) of malodorous compound, and this issealed in a 5 ml test vessel. This is allowed to stand at 23° C. for 12hours, and the content of malodorous compound in the vapor space abovethe liquid is determined quantitatively against a blank using headspaceGC.

Test Method 4

[0088] The silicon content of the absorbent polymers is determined byreacting silicate to form molybdenum blue and subsequent photometricanalysis. Previously, the silicon has been reacted quantitatively toform silicate, using alkaline decomposition (Photometrische Analyse,Authors: B. Lange, Zdenek, J. Vejdelek, S., edition of 1987, p. 383,VCH).

EXAMPLES cl Example 1a

[0089] This Example illustrates the production of a polymer gel havingsuperabsorbent properties.

[0090] A solution of 1300 g of acrylic acid, 2115.9 g of distilledwater, 2.7 g of polyethylene glycol monoallyl ether acrylate, and 1.25 gof polyethylene glycol diacrylate is prepared. Using 899.10 g of 50%sodium hydroxide solution, partial neutralization (degree ofneutralization (DN): 60%) is effected with stirring and cooling. Thesolution is cooled to 7-8° C. and purged with nitrogen for about 20minutes. Thereafter, 0.45 g of azobis(2-amidinopropane) dihydrochloridedissolved in 22.5 g of distilled Water, 1.35 g of sodiumperoxodisulfate, dissolved in 25 g of distilled water, and 0.315 g ofhydrogen peroxide (35%), dissolved in 22.5 g of distilled water, areadded. Subsequently, the polymerization is initiated by adding 0.0675 gof ascorbic acid dissolved in 9 g of water, whereupon a significant risein temperature occurs. A gel-like product is obtained, the furtherprocessing of which will be described in the following Examples.

Example 1b

[0091] This Example illustrates the production of another polymer gelhaving superabsorbent properties.

[0092] A solution of 1300 g of acrylic acid, 2015.9 g of distilledwater, 6.5 g of polyethylene glycol monoallyl ether acrylate, and 3.9 gof polyethylene glycol diacrylate is prepared. Using 997.10 g of 50%sodium hydroxide solution, partial neutralization (DN=70%) is effectedwith stirring and cooling. The solution is cooled to 7-8° C. and purgedwith nitrogen for about 20 minutes. Thereafter, 0.45 g ofazobis(2-amidinopropane) dihydrochloride dissolved in 22.5 g ofdistilled Water, 1.35 g of sodium peroxodisulfate, dissolved in 25 g ofdistilled water, and 0.315 g of hydrogen peroxide (35%), dissolved in22.5 g of distilled water, are added. Subsequently, the polymerizationis initiated by adding 0.0675 g of ascorbic acid dissolved in 9 g ofwater, whereupon a significant rise in temperature occurs. A gel-likeproduct is obtained, the further processing of which will be describedin the following Examples.

Example 1c

[0093] This Example illustrates the production of another polymer gelhaving superabsorbent properties.

[0094] A solution of 1300 g of acrylic acid, 2017.19 g of distilledwater, and 3.9 g of triallylamine as crosslinker is prepared. Using997.10 g of 50% sodium hydroxide solution, partial neutralization(DN=70%) is effected with stirring and cooling. The solution is cooledto 7-8° C. and purged with nitrogen for about 20 minutes. Thereafter,0.45 g of azobis(2-amidinopropane) dihydrochloride dissolved in 22.5 gof distilled Water, 1.35 g of sodium peroxodisulfate, dissolved in 25 gof distilled water, and 0.315 g of hydrogen peroxide (35%), dissolved in22.5 g of distilled water, are added. Subsequently, the polymerizationis initiated by adding 0.0675 g of ascorbic acid dissolved in 9 g ofwater, whereupon a significant rise in temperature occurs. A gel-likeproduct is obtained, the further processing of which will be describedin the following Examples.

Example 2

[0095] 500 g of the gels obtained in Examples 1b-c are willowed andsprayed uniformly with a suspension of Flavith® S108 (Degussa AG,SiO₂/Al₂O₃ ratio about 500) or Flavith® D (Degussa AG, SiO₂/Al₂O₃ ratioabout 56) or zeolite A (SiO₂/Al₂O₃ ratio <5) in water in amounts asspecified in the following Table, and subsequently dried to a residualwater content of <10% at 150° C. in a circulating air oven. Examples eand f are Comparative Examples because the zeolite A that is employed isnot a zeolite high in silicon. Retention Flavith S108 Flavith D ZeoliteA Example [g/g] wt.- % wt.- % wt.- % 2a 32.5* 1 0 0 2b 32.5* 2 0 0 2c33.0* 0 1 0 2d 33.0* 0 2 0  2e* 31.0^(#) 0 0 1  2f** 30.8^(#) 0 0 2

Example 3

[0096] In this Example, the retention and liquid absorption underpressure of a polymer product having superabsorbent properties areexamined in the absence of zeolite.

[0097] 50 g of the dried and milled polymer from Examples 1a-c screenedto 150-850 μm is wetted with a solution of 0.5 g of ethylene carbonateand 1.5 g of water in a plastic vessel with vigorous stirring and mixedthoroughly using a commercially available household hand mixer (Krupscompany). Subsequently, the wetted polymer is heated in an oven at atemperature of 180° C. for 30 minutes. Retention AAP Example [g/g] [g/g]3a 32.0 22.5 3b 28.0 24.5 3c 27.0 23.5

Example 4

[0098] In this Example, surface crosslinking is effected subsequent toadding zeolite.

[0099] 50 g of each dried and milled polymer from Examples 2a-f screenedto 150-850 μm is wetted separately with a solution of ethylene carbonate(EC) and water in a plastic vessel with vigorous stirring and mixedthoroughly using a commercially available household hand mixer (Krupscompany). The solution contains 0.25 g of EC per 1.8 g of water.Subsequently, the wetted polymer is heated in an oven at a temperatureof 170° C. for 30 minutes. Retention AAP Example [g/g] [g/g] 4a 27.524.5 4b 27.5 24.5 4c 28.0 24.0 4d 27.5 24.0  4e** 27 23.5  4f** 26.523.5

Example 5

[0100] In this Example, the zeolite is added during surfacecrosslinking.

[0101] 50 g of willowed, dried and milled polymer from Example 1ascreened to 150-850 μm is wetted in a plastic vessel with a solution of0.25 g of ethylene carbonate and 1.8 g of water and a suspension ofFlavith® S108 (Degussa AG) in amounts as specified in the followingTable (given in % dry substance relative to acrylic acid) with vigorousstirring and mixed thoroughly using a commercially available householdhand mixer (Krups company). Subsequently, the wetted polymer is heatedin an oven at a temperature of 180° C. for 30 minutes. Retention AAPFlavith S108 Example [g/g] [g/g] wt.- % 5a 30.6 22.5 1 5b 29.7 21.8 2

[0102] As is clearly recognized, the retention or liquid absorptionunder pressure, as compared to Example 3, has changed only slightlywithin the scope of the measuring precision.

Comparative Example 3a

[0103] (Analogous to WO 91/12029)

[0104] 10 g of methylcellulose (Walocel VP-M 20678) is dispersed with 40g of Flavith® S108 and 190 g of water using a high speed mixer,subsequently mixed with 50 g of a commercially available superabsorber(Favor® SXM 6860 by the Stockhausen company) in a laboratory mixer, anddried in a fluid-bed dryer at 60° C. in a constant air flow for 20minutes.

Comparative Example 3b

[0105] (Analogous to WO 91/12029)

[0106] 0.25 g of methylcellulose (Walocel VP-M 20678) is dispersed with1 g of Flavith® S108 and 5 g of water using a high speed mixer,subsequently mixed with 50 g of a commercially available superabsorber(Favor® SXM 6860 by the Stockhausen company) in a laboratory mixer, anddried in a fluid-bed dryer at 60° C. in a constant air flow for 20minutes. TB AAP Designation [g/g] [g/g] C3a 20.5 9.0 C3b 29.0 18.4 SXM6860 31.0 24.0

[0107] In addition to a dramatically deteriorated performance,particularly in the absorptive capacity under pressure compared to SXM6860, the instability of the composite material can already be seen in aheavy formation of dust when mixing or conveying the material.

Comparative Example 4

[0108] In this example, the zeolite is added subsequent to surfacecrosslinking.

[0109] 50 g of the product obtained in Example 4 is sprayed withthorough mixing with a suspension of zeolite (Flavith® S108,Degussa-Hüls company) in water in the amounts specified in the followingTable. The product is dried to a residual water content of <4% in adrying oven. Flavith S108 Designation [%] C4a 2 C4b 1

Example 6

[0110] The product obtained in Comparative Examples 3 and 4 is screened,and the fraction having a grain size of 150-850 μm is subjected to aball mill stability test wherein the product is stressed for 6 minutesat 95 rpm in the ball mill. Likewise, the product obtained in Example 2bis subjected to the same ball mill stability test. Again, the productsare screened, and the fraction having a grain size of <150 μm isexamined for its silicon content using test method 4. As the zeolitethat is employed has a grain size markedly below 150 μm, this methodallows a determination of the amount of zeolite that has been bound toor incorporated in the polymer having superabsorbent properties. Thefollowing quantities of silicon, relative to total dry substance, arefound in the samples: Theoretical Product from Ball mill SiO₂ contentSiO₂ content* Example test [%] [%] Comp. Ex. according Yes 5.2 2 toExample 1 of WO 91/12031** Comp. Ex. C3a No 79.9 40 Comp. Ex. C3a Yes79.1 40 Comp. Ex. C3b No 5.3 2 Comp. Ex. C3b Yes 3.2 2 2b Yes 1.7 2Comp. Ex. C4a Yes 2.9 2

[0111] As is clearly recognized, the products of the invention in thefraction of particles <150 μm have significantly less silicon comparedto products produced according to prior art, providing evidence thatbinding of the zeolites to the polymer is significantly stronger in theproducts according to the invention. Similarly, the composite materialsproduced following WO 91/12029 and WO 91/12031 are significantly moreunstable compared to the products of the invention, as evidenced by theincreased SiO₂ content in the fine dust prior to and subsequent to theball mill stability test. In particular, it is apparent that prior artprocesses are not capable of effectively binding such high amounts ofzeolite because a high percentage of the hydrophobic zeolite still isseparated from the superabsorber material after production. Whensubjecting the material to mechanical stress, large amounts of zeoliteare removed additionally, as characterized by the high silicon dioxidecontent in the fine dust after stressing in the ball mill. ComparativeExample C4a demonstrates that, even when using small amounts of zeolite(2%), binding in prior art processes is significantly poorer compared tothe process according to the invention (Ex. 2b).

Example 7

[0112] This Example examines the reduction in the vapor spaceconcentration of malodorous compounds.

[0113] In the measurement of malodorous substances, a polymer with nozeolite is examined as a blank according to test procedure 2, and thevapor space concentration of malodorous substance found is set as 100%.Subsequently, samples containing zeolite are examined, and the vaporspace concentration of malodorous substance is determined. The figuresin the right column are calculated as follows: 100×(detected amount ofodorous substance from polymer containing zeolite/detected amount ofodorous substance from polymer free of zeolite).

[0114] Doping with furfurylmercaptane and ethylfuran: Reduction offurfuryl- Reduction of ethyl- Polymer from mercaptane concentrationfuran concentration in Example in the vapor space [%] the vapor space[%] 4a 69.0 82.0 4b 89.0 93.0 4c 40 — 4d 66 — 4e Reduction <5% Reduction<5% 4f Reduction <5% Reduction <5% 5g 91.9 — 5h 99.1 —

[0115] The absorbent polymer products of the invention exhibit asignificant reduction of malodorous substances.

[0116] As shown by the Comparative Examples, zeolites having a lowsilicon dioxide/aluminum oxide ratio do not result in a satisfactoryreduction in the vapor space concentration of malodorous substances.

[0117] In order to have an additional confirmation, a commerciallyavailable FAVOR® superabsorber by Stockhausen company is mixedintimately with 2 wt.-% of various hydrophilic zeolites (A, P, X) havingan SiO₂/Al₂O₃ ratio of <5, and the reduction of odorous substances isexamined.

[0118] Compared to a sample with no zeolite, no reduction of theconcentration of odorous substances in the vapor space above the sampleis found.

1. An absorbent, crosslinked polymer for water or aqueous body fluids,based on optionally partially neutralized, monoethylenically unsaturatedmonomers bearing acid groups, characterized in that the polymer haszeolites high in silicon at least partially bound ionically thereto orincorporated therein.
 2. The polymer according to claim 1, characterizedin that the polymer includes and at least 0.01-10 wt.-%, preferably0.1-5 wt.-%, and more preferably 0.7-3 wt-% of zeolites high in silicon.3. The polymer according to claim 1 or 2, characterized in that thezeolite high in silicon has a silicon dioxide/aluminum oxide ratio inits skeleton of >20, preferably >50, more preferably >100, with >500being particularly preferred.
 4. The polymer according to any of claims1 to 3, characterized in that the polymer is constituted up to 30% offurther monoethylenically unsaturated monomers other than the monomersbearing acid groups.
 5. The polymer according to any of claims 1 to 4,characterized in that the polymer has 40 wt.-% of a water-soluble,natural or synthetic polymer incorporated therein by polymerizationand/or graft polymerization.
 6. The polymer according to any of claims 1to 5, characterized in that the polymer has been coated with 0.01-30wt.-%, preferably 0.1-10 wt.-%, relative to the polymer, of acrosslinker component which reacts with at least two carboxylic groupsin the surface layer of the polymer particles, thereby effectingcrosslinking.
 7. A process for producing the polymers according to anyof claims 1 to 6 by free-radical polymerization of an aqueous solutionof ethylenically unsaturated, optionally partially neutralized monomersbearing acid groups, optionally up to 30 wt.-% of furthermonoethylenically unsaturated comonomers, crosslinking monomers, andoptionally up to 40 wt.-% of a water-soluble natural or syntheticpolymer according to the process of solution or suspensionpolymerization to form a hydrogel, optional isolation, crushing,followed by drying, milling/screening, and secondary surfacecrosslinking, characterized in that the zeolite high in silicon is addedto the polymer during its surface crosslinking at the latest, preferablyin suspension.
 8. A process for producing the polymers according to anyof claims 1 to 7 by free-radical polymerization of an aqueous solutionof ethylenically unsaturated, optionally partially neutralized monomersbearing acid groups, optionally up to 30 wt.-% of furthermonoethylenically unsaturated comonomers, crosslinking monomers, andoptionally up to 40 wt.-% of a water-soluble natural or syntheticpolymer according to the process of solution or suspensionpolymerization to form a hydrogel, optional isolation, crushing,followed by drying, milling/screening, characterized in that the zeolitehigh in silicon is added to the polymer at a water content of >10 wt.-%,preferably in suspension.
 9. The process according to claim 8,characterized in that the water content is >30, preferably >50, and morepreferably >65 wt.-%.
 10. use of the polymers according to any of claims1 to 6 for improved absorption of odors from body fluids.
 11. Use of thepolymers according to any of claims 1 to 6 as an absorbent for aqueousliquids, preferably in constructions for absorbing body fluids, infoamed and non-foamed sheet materials, in packaging materials, in plantbreeding, and as soil improver.
 12. Use of the polymers according to anyof claims 1 to 6 in hygiene articles.
 13. Use of the polymers accordingto any of claims 1 to 6 as a vehicle and/or stabilizer for activesubstances, particularly fertilizers or other active substances beingreleased optionally in a delayed fashion.